Two processes, the de novo formation of vessels from differentiating endothelial cells or angioblasts in the developing embryo (vasculogenesis) and the growth of new capillary vessels from existing blood vessels (angiogenesis), are involved in the development of the vascular systems of animal organs and tissues, as well as in transitory phases of angiogenesis, for example during the menstrual cycle, in pregnancy, or in wound healing. On the other hand, a number of diseases are known to be associated with deregulated angiogenesis, for example diseases caused by ocular neovascularisation, such as retinopathies (including diabetic retinopathy), age-related macula degeneration, psoriasis, haemangioblastoma, haemangioma, an inflammatory disease, such as a rheumatoid or rheumatic inflammatory disease, especially arthritis, such as rheumatoid arthritis, and especially neoplastic diseases, for example so-called solid tumours and liquid tumours (such as leucemias).
Recent findings show that at the centre of the network regulating the growth and differentiation of the vascular system and its components, both during embryonic development and normal growth and in a wide number of pathological anomalies and diseases, lies the angiogenic factor known as “Vascular Endothelial Growth Factor” (=VGEF), along with its cellular receptors (see Breier, G., et al., Trends in Cell Biology 6, 454-6 [1996] and the references cited therein).
VEGF is a dimeric, disulfide-linked 46-kDa glycoprotein and is related to “Platelet-Derived Growth Factor” (PDGF). It is produced by normal cell lines and tumour cell lines, is an endothelial cell-specific mitogen, shows angiogenic activity in in vivo test systems (e.g. rabbit-cornea), is chemotactic for endothelial cells and monocytes, and induces plasminogen activators in endothelial cells, which are then involved in the proteolytic degradation of extracellular matrix during the formation of capillaries. A number of isoforms of VEGF are known which show comparable biological activity, but differ in the type cells that secrete them and in their heparin-binding capacity. In addition, there are other members of the VEGF family, such as “Placenta Growth Factor” (PLGF) and VEGF-C.
VEGF receptors, however, are transmembranous receptor tyrosine kinases and have an extracellular domain with seven immunoglobulin-like domains and an intracellular tyrosine kinase domain. Various types are known, e.g. VEGFR-1. VEGFR-2, and VEGFR-3.
A large number of human tumours, especially gliomas and carcinomas, express high levels of VEGF and its receptors. This has led to the hypothesis that the VEGF released by tumour cells could stimulate the growth of blood capillaries and the proliferation of tumour endothelium in a paracrine manner and thus, through the improved blood supply, accelerate tumour growth. Increased VEGF expression could explain the occurrence of cerebral oedema in patients with glioma. Direct evidence of the role of VEGF as a tumour angiogenesis factor in vivo has been obtained from studies in which VEGF expression or VEGF activity was inhibited. This was achieved with antibodies which inhibit VEGF activity, with dominant-negative VEGFR-2 mutants which inhibited signal transduction, or with the use of antisense-VEGF RNA techniques. All approaches led to a reduction in the growth of glioma cell lines or other tumour cell lines in vivo as a result of inhibited tumour angiogenesis.
Hypoxia and also a large number of growth factors and cytokines, e.g. Epidermal Growth Factor, Transforming Growth Factor α, Transforming Growth Factor β, Interleukin 1, and Interleukin 6, induce the expression of VEGF in cell experiments. Angiogenesis is regarded as an absolute prerequisite for those tumours which grow beyond a maximum diameter of about 1-2 mm; up to this limit, oxygen and nutrients may be supplied to the tumour cells by diffusion. Every tumour, regardless of its origin and its cause, is thus dependent on angiogenesis for its growth after it has reached a certain size.
Three principal mechanisms play an important part in the anti-tumour activity of angiogenesis inhibitors: 1) Inhibition of the growth of vessels, especially capillaries, into avascular resting tumours, with the result that there is no net tumour growth owing to the balance that is achieved between apoptosis and proliferation; 2) Prevention of the migration of tumour cells owing to the absence of bloodflow to and from tumours; and 3) Inhibition of endothellal cell proliferation, thus avoiding the paracrine growth-stimulating effect exerted on the surrounding tissue by the endothellal cells which normally line the vessels.